The non-invasive cryoablation of tissue within the vasculature (e.g. the veins, arteries and chambers of the heart) of a patient can be used to effectively destroy or isolate diseased tissue. For example, atrial fibrillation, which is a somewhat common heart condition, can be treated by cryoablating a circumferential band of tissue surrounding the ostium where a pulmonary vein connects with the left atrium to prevent abnormal electrical signals from reaching the heart. To perform such a procedure, the tip of a cryoablation catheter is typically inserted into and advanced within the vasculature of a patient until the tip is located adjacent to the targeted tissue. Next, in a typical cryocatheter, a refrigerant is pumped into the catheter for expansion into an expansion chamber that is located at or near the catheter tip. The expansion of the refrigerant cools the catheter tip and target tissue.
One way to monitor and control the expansion of a refrigerant near the distal tip of a cryocatheter is to monitor the pressure within the expansion chamber. In particular, the measured pressure within the chamber provides an indication of the flow of refrigerant through the tip, which in turn, provides an indication of the instantaneous cooling power of the cryocatheter. In addition, for a cryoablation system in which the refrigerant undergoes a phase change during expansion (i.e. transitions from a liquid to a gaseous state), the measured chamber pressure provides an indication of the actual refrigerant boiling temperature. This boiling temperature provides a direct indication of the temperature of the cryoablation catheter tip. Moreover, the measured chamber pressure can be used continuously during an ablation procedure to vary the flow of refrigerant into the catheter to optimize both the tip temperature and the catheter's cooling power.
With the above in mind, there are several drawbacks associated with placing a pressure sensor directly in the expansion chamber of a cryocatheter. First, such a positioning scheme uses up critical space at the distal tip of the catheter. More specifically, placing the pressure sensor at the tip results in either a reduction in expansion chamber volume or an increase in catheter tip size. The former can cause a reduction in refrigerant flow which can effectively lower the cooling power of the cryocatheter. On the other hand, increasing the tip size reduces the likelihood that the catheter tip can successfully navigate through the vasculature and reach the treatment site. As is well known, the human vasculature is curved, branched and contains vessels having relatively small inner diameters. As a consequence, it is necessary to design a catheter having a relatively low profile to allow the distal end of the catheter to navigate through the complex vasculature.
In addition to space considerations, the expansion chamber presents a relatively harsh environment for a pressure sensor. Specifically, a sensor positioned in the expansion chamber must be operable over a wide range of temperatures, including cryogenic temperatures as low as minus 85 degrees C., or lower.
In light of the above, it is an object of the present invention to provide systems and methods for measuring the pressure within an expansion chamber at the distal end of a cryocatheter which do not require a reduction in the size of the chamber or an increase in the size of the catheter tip. It is still another object of the present invention to provide a system for measuring an expansion chamber pressure with pressure sensors that remain positioned at extracorporeal locations during a cryocatheter procedure. Yet another object of the present invention is to provide systems and methods for monitoring the temperature and cooling power of a cryocatheter tip which are easy to use, relatively simple to implement, and comparatively cost effective.